The vast majority of semiconductor discrete devices and integrated circuits are fabricated from silicon, at least in part because of the availability of inexpensive, high quality monocrystalline silicon substrates. Other semiconductor materials, such as the so-called compound semiconductor materials, have physical attributes, including wider bandgap and/or higher mobility than silicon, or direct bandgaps that make these materials advantageous for certain types of semiconductor devices. Unfortunately, compound semiconductor materials are generally much more expensive than silicon and are not available in large wafers as is silicon. Gallium arsenide (GaAs), the most readily available compound semiconductor material, is available in wafers only up to about 150 millimeters (mm) in diameter. In contrast, silicon wafers are available up to about 300 mm and are widely available at 200 mm. The 150 mm GaAs wafers are many times more expensive than are their silicon counterparts. Wafers of other compound semiconductor materials are even less available and are more expensive than GaAs.
Because of the desirable characteristics of compound semiconductor materials, and because of their present generally high cost and low availability in bulk form, for many years attempts have been made to grow thin films of the compound semiconductor materials on a foreign substrate. To achieve optimal characteristics of the compound semiconductor material, however, a monocrystalline film of high crystalline quality is desired. Attempts have been made, for example, to grow layers of a monocrystalline compound semiconductor material on germanium, silicon, and various insulators. These attempts have generally been unsuccessful because lattice mismatches between the host crystal and the grown crystal have caused the resulting thin film of compound semiconductor material to be of low crystalline quality.
If a large area thin film of high quality monocrystalline compound semiconductor material was available at low cost, a variety of semiconductor devices could advantageously be fabricated in that film at a low cost compared to the cost of fabricating such devices on a bulk wafer of compound semiconductor material or in an epitaxial film of such material on a bulk wafer of compound semiconductor material. In addition, if a thin film of high quality monocrystalline compound semiconductor material could be realized on a bulk wafer such as a silicon wafer, an integrated device structure, such as, for example, a resonant heterojunction tunnel diode, could be achieved that took advantage of the best properties of both the silicon and the compound semiconductor material.
Tunnel diodes, where carriers tunnel through the band gap of a doped p-n junction, have taken many forms since first proposed in about 1958. Tunnel diodes provide very fast switching time and low power dissipation. The first tunnel diode, called an Esaki tunneling diode (after its originator), comprised two silicon regions of different conductivity types with both being highly doped. When bias is applied to the Esaki-type diode, the available states for electrons in the contact layer align with available states for holes in the valence band of the injection layer and tunneling occurs. Traditional Esaki diodes formed in silicon-based material systems, however, exhibit low peak current densities, low peak-to-valley current ratios, and low operational frequencies, which make them unsuitable for present-day demands for enhanced performance in areas such as high frequency circuits, portable communications systems, and digital applications. Thus, present-day Esaki-type diodes may comprise germanium, gallium arsenide, or other semiconductor materials, which exhibit more suitable electrical properties. For example, tunnel diodes having the most favorable electrical properties are currently manufactured in compound semiconductor material systems comprising materials such as indium aluminum arsenide (InAlAs), indium gallium arsenide (InGaAs), indium phosphide (InP), gallium arsenide (GaAs), and aluminum arsenide (AlAs). Unfortunately, as stated above, such compound semiconductor substrates tend to be expensive and extremely fragile.
Accordingly, a need exists for a semiconductor structure that provides a high quality monocrystalline compound semiconductor film over another monocrystalline material and for a process for making such a structure.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.